A raft of fresh prison closures will see some 2,600 inmate places cut in Britain, the Justice Secretary has said as he unveiled tentative plans for Britain's biggest jail.

Six prisons will shut by the end of March and three jails will be partially closed as part of a far-reaching overhaul that could put more than 1,000 jobs at risk.

The drop in inmate places will be offset by four new mini-prisons, with room for 1,260 offenders, while a feasibility study for a 2,000 capacity super-prison was launched.

Chris Grayling said the plans will save the country £63 million a year as the cost per prisoner in new facilities is much lower than older sites. But the Public and Commercial Services (PCS) union said the closures were "irresponsible" and campaigners dubbed plans for a super-prison a "gigantic mistake."

The proposals for the super-prison appear to contrast with the views of Mr Grayling's predecessor Kenneth Clarke, who claimed jails were expensive.

Mr Grayling said: "We have to bring down the cost of our prison system, much of which is old and expensive. But I never want the courts to be in a position where they cannot send a criminal to prison because there is no place available. So we have to move as fast as we can to replace the older parts of our prison system."

Among the prisons to be closed is Britain's oldest jail, HMP Shepton Mallet, Somerset, which traces its roots back to 1610 and housed important documents such as the Magna Carta during the Second World War.

Other prisons marked for closure include Bullwood Hall in Essex, Canterbury in Kent, Gloucester, Kingston in Portsmouth and Shrewsbury in Shropshire, while Chelmsford, Hull and the Isle of Wight will see some accommodation partly cut.

PCS general secretary Mark Serwotka said: "These closures are unnecessary, irresponsible and amount to more privatisation by stealth. The fact this is happening without any public debate or discussion ought to be a national scandal and we urgently need an independent review to look at the impact of prison privatisation on our communities, staff and prisoners."

Shadow justice secretary Sadiq Khan said there were "no replacement places guaranteed in the short term" for the closures announced. He said: "The public will want reassurance that there's enough prison places over the coming years to keep safely behind bars those found guilty of serious crimes, and that enough is being done to rehabilitate and reform prisoners to stop them re-offending."